1. Field of the Invention
The present invention relates to optical coherence tomography. More specifically, the present invention relates to methods and systems for analyzing the thickness of a tear film layer and the heights of tear menisci around upper and lower eyelids of an eye.
2. Description of the Related Art
Tears secreted by the lacrimal gland spread over the cornea with each blink to form a thin layer (i.e., tear film) and menisci at the upper and lower eyelids. The volume and distribution of the tear film layer and the upper and lower menisci change rapidly between successive blinks. The dynamic variation of the tear film layer and upper and lower menisci plays an important role in visual function, maintenance of corneal integrity and ocular comfort. For example, dry eye syndrome (DES) has been reported to affect functional visual acuity and the performance of routine activities, such as working, reading and driving. DES is characterized by symptoms of ocular dryness and discomfort due. However, there are currently no objective diagnostic criteria for DES and no objective means of evaluating treatment efficacy. Indeed, the dynamic variations of tears remain one of the most neglected areas of ophthalmology.
This neglect is due in part to the difficulty of quantitatively measuring the thickness of the tear film layer. The neglect is also partly due to the difficulty of quantitatively imaging the tear menisci around both upper and lower eyelids during a period of one or more successive blinks when the tear is redistributed around the cornea and upper and lower eyelids. Since the tear film characteristics vary widely from person to person and are strongly influenced by both physiological and environmental conditions, it is difficult to characterize these tear parameters using a single method.
Objective measures of tear film have been elusive for several reasons including the difficulty of achieving acceptable optical resolution of tear film and the fact that tear film is continuously changing due to tear drainage and evaporation. Contact methods for measuring tear volume using, for example, a glass fiber or filter paper are associated with a risk of injury and are not usually applied on human subjects. Further, such methods provide static measurements and do not provide information on how the distribution and volume of tears change over time.
Optical coherence tomography (OCT) is a noninvasive, non-contact imaging technology that has been used to image the anterior segment of the eye, including thickness measurements of the retina and nerve fiber layers. OCT has also been used to measure the thickness of the cornea, epithelium, corneal flap and iris. OCT generates two-dimensional, cross-sectional images from multiple scans of back-scattered light. OCT has good repeatability in the measurement of corneal thickness.
Since tear fluid and ocular structures have different refractive indices, OCT has been used in a limited way to study tears. However, OCT devices previously used to analyze tear film only generate approximately one image per second and are typically reset between images such that only one image can be acquired approximately every twenty seconds. Such slow acquisition rates are not suitable for measuring the rapid changes in tear dynamics.
Further, OCT tear measurements typically measure only one tear parameter at a time. For example, an OCT image typically provides information for only the lower meniscus or for the tear film thickness, but not both. Thus, OCT tear film measurements have not been used to analyze the relationships between tear film thickness and meniscus height around both upper and lower eyelids.